Added Extrude!

I've fully implemented the extrude tool. Just hold down the hotkey and move the mouse to extrude the selected faces.

Features:

• Interactive (use mouse to change distance)
• Works in Face, Vertex, and Edge select modes.

Implementation Details: It was a little tricker getting it to work in vertex and edge mode than expected.  For example, in vertex mode, it will extrude all faces that have all of its vertices selected. But when extrude splits the edges, both halves of each edge are still technically selected. So the translate function tried to move both edges (resulting in no extrude occuring, visually). To fix it, I just deselect all before extruding, then reselect the boundary edges, using SelectPrimitive, which selects the vertices correctly based on the current mode. Sorry if I overexplained. I just wanted to share an example of how things can be trickier than they seem.